Re: n_tty: Check the other end of pty pair before returning EAGAIN on a read()
From: Brian Bloniarz
Date: Sun Feb 28 2016 - 17:58:07 EST
Hi Peter, I saw Marc Aurele La France's proposed patch to n_tty to fix
OpenSSH, and your feedback. Patch below is an attempt to address that
feedback. Please let me know if this is the change you envisioned;
(see Marc's excellent original writeup for details on the issue).
[PATCH] n_tty: wait for buffer work in read() and poll().
Undoes the following four changes:
1) f95499c3030fe1bfad57745f2db1959c5b43dca8
n_tty: Don't wait for buffer work in read() loop
2) f8747d4a466ab2cafe56112c51b3379f9fdb7a12
tty: Fix pty master read() after slave closes
3) 52bce7f8d4fc633c9a9d0646eef58ba6ae9a3b73
pty, n_tty: Simplify input processing on final close
4) 1a48632ffed61352a7810ce089dc5a8bcd505a60
pty: Fix input race when closing
These changes caused a regression in OpenSSH, as it assumes that the
first read() to return EAGAIN after a SIGCHLD means that all the child's
output has been returned.
Inspired by analysis and patch from Marc Aurele La France <tsi@xxxxxxxxxx>
Reported-by: Volth <openssh@xxxxxxxxx>
Reported-by: Marc Aurele La France <tsi@xxxxxxxxxx>
BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=52
BugLink: https://bugzilla.mindrot.org/show_bug.cgi?id=2492
Signed-off-by: Brian Bloniarz <brian.bloniarz@xxxxxxxxx>
---
Documentation/serial/tty.txt | 3 ---
drivers/tty/n_hdlc.c | 2 +-
drivers/tty/n_tty.c | 34 +++++++++++++++-------------------
drivers/tty/pty.c | 4 +---
drivers/tty/tty_buffer.c | 29 +----------------------------
include/linux/tty.h | 1 -
6 files changed, 18 insertions(+), 55 deletions(-)
diff --git a/Documentation/serial/tty.txt b/Documentation/serial/tty.txt
index bc3842d..e2dea3d 100644
--- a/Documentation/serial/tty.txt
+++ b/Documentation/serial/tty.txt
@@ -213,9 +213,6 @@ TTY_IO_ERROR If set, causes all subsequent userspace read/write
TTY_OTHER_CLOSED Device is a pty and the other side has closed.
-TTY_OTHER_DONE Device is a pty and the other side has closed and
- all pending input processing has been completed.
-
TTY_NO_WRITE_SPLIT Prevent driver from splitting up writes into
smaller chunks.
diff --git a/drivers/tty/n_hdlc.c b/drivers/tty/n_hdlc.c
index bbc4ce6..c35abef 100644
--- a/drivers/tty/n_hdlc.c
+++ b/drivers/tty/n_hdlc.c
@@ -600,7 +600,7 @@ static ssize_t n_hdlc_tty_read(struct tty_struct *tty, struct file *file,
add_wait_queue(&tty->read_wait, &wait);
for (;;) {
- if (test_bit(TTY_OTHER_DONE, &tty->flags)) {
+ if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
ret = -EIO;
break;
}
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index b280abaa..fc04011 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1952,10 +1952,20 @@ err:
return -ENOMEM;
}
+/**
+ * Synchronously pushes the terminal flip buffers to the line discipline
+ * and checks for available data.
+ *
+ * Must not be called from IRQ context.
+ */
static inline int input_available_p(struct tty_struct *tty, int poll)
{
struct n_tty_data *ldata = tty->disc_data;
- int amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
+ int amt;
+
+ flush_work(&tty->port->buf.work);
+
+ amt = poll && !TIME_CHAR(tty) && MIN_CHAR(tty) ? MIN_CHAR(tty) : 1;
if (ldata->icanon && !L_EXTPROC(tty))
return ldata->canon_head != ldata->read_tail;
@@ -1963,18 +1973,6 @@ static inline int input_available_p(struct tty_struct *tty, int poll)
return ldata->commit_head - ldata->read_tail >= amt;
}
-static inline int check_other_done(struct tty_struct *tty)
-{
- int done = test_bit(TTY_OTHER_DONE, &tty->flags);
- if (done) {
- /* paired with cmpxchg() in check_other_closed(); ensures
- * read buffer head index is not stale
- */
- smp_mb__after_atomic();
- }
- return done;
-}
-
/**
* copy_from_read_buf - copy read data directly
* @tty: terminal device
@@ -2170,7 +2168,7 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
struct n_tty_data *ldata = tty->disc_data;
unsigned char __user *b = buf;
DEFINE_WAIT_FUNC(wait, woken_wake_function);
- int c, done;
+ int c;
int minimum, time;
ssize_t retval = 0;
long timeout;
@@ -2238,10 +2236,8 @@ static ssize_t n_tty_read(struct tty_struct *tty, struct file *file,
((minimum - (b - buf)) >= 1))
ldata->minimum_to_wake = (minimum - (b - buf));
- done = check_other_done(tty);
-
if (!input_available_p(tty, 0)) {
- if (done) {
+ if (test_bit(TTY_OTHER_CLOSED, &tty->flags)) {
retval = -EIO;
break;
}
@@ -2445,12 +2441,12 @@ static unsigned int n_tty_poll(struct tty_struct *tty, struct file *file,
poll_wait(file, &tty->read_wait, wait);
poll_wait(file, &tty->write_wait, wait);
- if (check_other_done(tty))
- mask |= POLLHUP;
if (input_available_p(tty, 1))
mask |= POLLIN | POLLRDNORM;
if (tty->packet && tty->link->ctrl_status)
mask |= POLLPRI | POLLIN | POLLRDNORM;
+ if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
+ mask |= POLLHUP;
if (tty_hung_up_p(file))
mask |= POLLHUP;
if (!(mask & (POLLHUP | POLLIN | POLLRDNORM))) {
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 2348fa6..6427a39 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -59,7 +59,7 @@ static void pty_close(struct tty_struct *tty, struct file *filp)
if (!tty->link)
return;
set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
- tty_flip_buffer_push(tty->link->port);
+ wake_up_interruptible(&tty->link->read_wait);
wake_up_interruptible(&tty->link->write_wait);
if (tty->driver->subtype == PTY_TYPE_MASTER) {
set_bit(TTY_OTHER_CLOSED, &tty->flags);
@@ -247,9 +247,7 @@ static int pty_open(struct tty_struct *tty, struct file *filp)
goto out;
clear_bit(TTY_IO_ERROR, &tty->flags);
- /* TTY_OTHER_CLOSED must be cleared before TTY_OTHER_DONE */
clear_bit(TTY_OTHER_CLOSED, &tty->link->flags);
- clear_bit(TTY_OTHER_DONE, &tty->link->flags);
set_bit(TTY_THROTTLED, &tty->flags);
return 0;
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 3cd31e0..3c0e914 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -37,29 +37,6 @@
#define TTY_BUFFER_PAGE (((PAGE_SIZE - sizeof(struct tty_buffer)) / 2) & ~0xFF)
-/*
- * If all tty flip buffers have been processed by flush_to_ldisc() or
- * dropped by tty_buffer_flush(), check if the linked pty has been closed.
- * If so, wake the reader/poll to process
- */
-static inline void check_other_closed(struct tty_struct *tty)
-{
- unsigned long flags, old;
-
- /* transition from TTY_OTHER_CLOSED => TTY_OTHER_DONE must be atomic */
- for (flags = ACCESS_ONCE(tty->flags);
- test_bit(TTY_OTHER_CLOSED, &flags);
- ) {
- old = flags;
- __set_bit(TTY_OTHER_DONE, &flags);
- flags = cmpxchg(&tty->flags, old, flags);
- if (old == flags) {
- wake_up_interruptible(&tty->read_wait);
- break;
- }
- }
-}
-
/**
* tty_buffer_lock_exclusive - gain exclusive access to buffer
* tty_buffer_unlock_exclusive - release exclusive access
@@ -254,8 +231,6 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
if (ld && ld->ops->flush_buffer)
ld->ops->flush_buffer(tty);
- check_other_closed(tty);
-
atomic_dec(&buf->priority);
mutex_unlock(&buf->lock);
}
@@ -505,10 +480,8 @@ static void flush_to_ldisc(struct work_struct *work)
*/
count = smp_load_acquire(&head->commit) - head->read;
if (!count) {
- if (next == NULL) {
- check_other_closed(tty);
+ if (next == NULL)
break;
- }
buf->head = next;
tty_buffer_free(port, head);
continue;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index d9fb4b0..af18365 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -338,7 +338,6 @@ struct tty_file_private {
#define TTY_EXCLUSIVE 3 /* Exclusive open mode */
#define TTY_DEBUG 4 /* Debugging */
#define TTY_DO_WRITE_WAKEUP 5 /* Call write_wakeup after queuing new */
-#define TTY_OTHER_DONE 6 /* Closed pty has completed input processing */
#define TTY_LDISC_OPEN 11 /* Line discipline is open */
#define TTY_PTY_LOCK 16 /* pty private */
#define TTY_NO_WRITE_SPLIT 17 /* Preserve write boundaries to driver */